Flat panel light sources comprise LEDs and light guide plates positioned adjacent to the LEDs. The light guide plate comprises a light incident surface facing the LEDs and a light emitting surface perpendicular to the light incident surface. When the LEDs emit light, the light guide plate can reflect the light towards the light emitting surface.
However, a portion of the light emitted by the LEDs may not enter the light guide plate, which may lower the light coupling efficiency of the flat panel light source. Thus, the thickness of the light guide plate may need to be increased. However, a thin-type flat panel light source cannot be obtained. Improvement in the art is preferred.